We propose to carry out detailed physical-chemical studies of cytochrome c1aa3 and the Rieske iron-sulfur protein isolated from the extremely thermophilic, aerobic bacterium, Thermus thermophilus. Preliminary studies have shown that the functional components of these proteins are identical to analogous proteins obtained from mitochondria. Our studies take advantage of bacterial metabolism to specifically introduce probe atoms into the molecules of interest, and our experiments will lead to new knowledge of the active-site structures of these proteins and how they function. The work proposed for this grant period is planned to supply information and materials from which to construct a "minimum component" respiratory system. It is suggested that proteins obtained from extreme thermophiles, because of their resistance to denaturation, are ideal subjects for detailed examination of mechanisms in respiration.